Methods for coating metallic substrates are known from the prior art.
DE 39 15 080 A1 describes a method for coating conductive substrates by immersing them into an aqueous electrocoat material which comprises at least one cathodically depositable synthetic resin. After a film has been electrophoretically deposited on the substrate, the deposited coating film is baked. An example of a cathodically depositable synthetic resin that can be used is a cationic, amine-modified epoxy resin. This resin either is self-crosslinking, in which case the amine-modified epoxy resin contains, for example, blocked isocyanate groups, or is externally crosslinking, in which case the aqueous electrocoat material comprises a suitable crosslinker component, such as a blocked polyisocyanate, for example. The coating is baked generally at temperatures from 130 to 200° C.
EP 0 941 295 A1 describes a method for coating substrates, preferably of metal, by cleaning the substrate surface, optionally applying a pretreatment composition, applying a primer, and applying a topcoat by spraying, dipping, knifing, rolling or spreading, the primer comprising an amine-modified epoxy resin. As in DE 39 15 080 A1, the amine-modified epoxy resin either may be self-crosslinking, in which case the amine-modified epoxy resin comprises, for example, blocked isocyanate groups, or is externally crosslinking, in which case the primer additionally comprises a suitable crosslinker component, such as a blocked polyisocyanate, for example. The coating is baked generally at temperatures from 90 to 300° C.
EP 0 272 525 A2 describes the preparation of partial condensation products from a film-forming polycarboxyl compound and an epoxy resin-amine adduct. The condensation products are combined for the formulation of water-thinable coatings with crosslinking components such as phenolic, melamine, urea, benzoguanamine or glycoluril resins. The coating films are oven-drying.
WO 95/26373 describes aqueous coating compositions for can or coil coatings, more particularly for the interior coating of beverage cans, the compositions comprising an epoxide-amine adduct and also a carboxyl-bearing vinyl polymer. There may be crosslinking agents present such as amino resins, phenolic resins, urea resins or blocked isocyanates. The coatings are cured typically at 200-300° C.
One of the key objectives in the coating of metallic substrates is to achieve effective corrosion control. This is the case, for example, both for automotive OEM finishing and for automotive refinish. In automotive OEM finishing, sufficient corrosion control may typically be achieved through passivation of the substrate, application and baking of appropriate corrosion-control coatings, such as electrocoats and primers, for example, and also further coating materials.
Automotive refinish differs from automotive OEM finishing essentially first in that the paint finish is not produced on the production line under corresponding industrial application conditions, and second in that it has to be carried out on a completed automobile with all of its components. Automotive refinish takes place in general with skilled-labor methods, by manual spray application, under drying and curing conditions of typically not more than 80° C. Higher temperatures, even if occurring only locally, may give rise to damage due to deformation or melting of plastics components, deformation of the tires, or irreversible defects caused to the electrics and electronics of the vehicles. It follows from this that the coating materials required for refinish are different than for OEM. In particular, the coating materials must be suitable for spray application and must cure completely, or substantially completely, even at temperatures of below 90° C. With the known coating materials as described above, this is not possible, and hence they are not suitable for automotive refinish.
Corrosion control therefore poses a particular challenge in the case of automotive refinish. Sufficient corrosion-control properties are presently achievable in refinish by using what are called acid primers (wash primers) based on polyvinyl butyral. The fraction of volatile organic constituents (VOC) in these wash primers, however, at about 700 g/L, is very high, and this is extremely undesirable from the standpoint of environmental protection and against the background of increasingly stringent legislation. The high VOC level is based on a high fraction of organic solvents in the known corrosion-control wash primers. Effective corrosion control is also achievable with chromate-containing primers. Considerations of environmental protection, however, have rendered such primers anachronistic. But with the existing modern aqueous primers for the refinish sector, adequate corrosion control is impossible to achieve.
The problem addressed by the present invention was therefore that of providing a method for coating metallic substrates that is suitable for automotive refinish, managing as far as possible without chromates and with minimal amounts of organic solvents, and yet at the same time allowing effective corrosion control to be achieved.
The resultant coatings ought, moreover, to exhibit effective adhesion to the substrate, and good intercoat adhesion with other coating films, and to contribute to high stone chip resistance on the part of the paint finish.
The method shall as far as possible also be suitable for the finishing of plastics substrates accordingly.